I noticed that YOU need to have a special retainer to PLAY and USE certain units in MP battles.

For example, to use Great Guard, Mangonels, and I think even Ninjas, the player has to place that retainer in their active retainer list other wise it will read "Invalid Error".

Also, if the error is caused by YOU. Where the check mark is it will read "Your". So if you see the any wording at all that has "Your" in in where the check mark is, that means your army is invalid some where and you must fix it.

If its one of the other players it will simply read "A player....."

Also, someone mentioned somewhere that you may not be able to play veterans if they are lower than 100%.